As described in the above-identified copending applications and the literature referred to or cited therein and in the same class, the problem of disposing, storing and transporting radioactive materials such as radioactive wastes obtained in nuclear power plant operations, such as irradiated fuel elements, generally requires that a vessel, canister or container be provided which can be hermetically sealed and is of a sufficient thickness to block the emission of radioactivity from the contents of the vessel to the ambient environment.
It has been proposed, for example, to provide relatively thick-walled vessels which can be cast from high-density materials providing a gamma-radiation shield, e.g. from cast iron, cast steel or spherolitic (i.e. spheroidal-graphite or nodular) cast iron, the cast structure having sufficient structural strength to withstand rough handling, little tendency to fracture, rupture or crack, and excellent gamma-radiation shielding effectiveness.
The generally upright containers, e.g. containers formed in one piece with a bottom and vertical walls, can be provided with a plug-type cover to prevent the escape of radiation in the end of the container which is closed and means can be provided to serve as a neutron absorber or moderator.
In general the latter means can consist of a material having a higher absorption cross section for neutrons.
For example, some of the above-identified copending applications describe constructions of a container for the purposes set forth in which passages are provided at least in the vertical walls of the vessel, e.g. during the casting thereof, into which a moderating material is introduced.
When reference is made herein to a cast vessel wall of cast iron or spherolytic cast iron, however, it should be understood that it does not exclude a matrix of the cast metal in which gamma-radiation absorbers are embedded. Any gamma-radiation absorbers conventional in the art, therefore, may be disposed in such a metal matrix within the purview of the present invention.
Neutron moderators can be identified herein as materials capable of braking the energy of neutrons to velocities which render the neutrons incapable of detrimentally affecting living organisms and nonliving systems.
In a transport and storage vessel which has been found to be particularly effective, e.g. as described in German patent document (Utility Model-Gebruuchsmuster) No. 77 27 690, the moderator passages extend vertically and are spaced apart about the periphery of the vessel with as close a spacing as possible and are given an elongated cross section to form, to the greatest extent feasible, a closed shield around the contents of the vessel. The elongated cross section was of oval or rectangular type and extended so that the major dimension of the cross section, the major axis of the ellipse, was tangential, secantial or otherwise offradial so that projections of the passages in the radial direction generally overlapped.
Even when the passages had other cross sections, the prevailing principle was to provide the moderator cross section as large as possible consistent with structural stability of the vessel walls and the space between the passages as close as possible also consistent with such structural stability.
As the passages are positioned more closely, or the volume of the passages is increased for a given wall thickness, the strength of the wall diminishes and hence thicker walls must be used for a given set of parameters in terms of passage cross section and spacing.
Furthermore, in the earlier vessel construction, the moderator-containing passages were generally located in a row separating inner and outer layers of the cast material from one another so that the connection between these layers was effected with relatively thin webs. This again was detrimental to the mechanical stability of the device.
The primary disadvantage was the inability of the vessel to withstand sudden shock as must be tolerated in transport and other handling of the container. Here again, the remedy was to increase the mass, wall thickness and size of the container to counter the decreased stability and thereby increase the handling complexity, transport cost and fabrication cost.